An example of a communication apparatus and communication method equipped with an array antenna is described in the Unexamined Japanese Patent Publication No. HEI 10-336149.
An array antenna is configured by a plurality of antenna elements and allows transmission directivity to be freely set by adjusting the amplitude and phase of a signal transmitted from each antenna element.
FIG. 1 is block diagram showing a configuration of the transmitting side of a base station apparatus equipped with a conventional array antenna.
Base station apparatus 1 has a configuration equipped with an array antenna made up of two antennas 2 and 3, radio apparatuses 4 and 5 with antennas 2 and 3 connected thereto, switch 6, measuring apparatus 7 and baseband signal processing apparatus 8. Baseband signal processing apparatus 8 has a configuration equipped with baseband signal generator 9, phase/amplitude correction sections 10 and 11 and error storage section 12. Radio apparatuses 4 and 5 have a configuration equipped with quadrature modulation sections 13 and 14, transmit power amplifiers 15 and 16 and switches 17 and 18, respectively.
However, base station apparatus 1 is normally equipped with a plurality of baseband signal processing apparatuses to generate transmission signals to a plurality of mobile station apparatuses, which are not shown and FIG.1 shows a case where only one baseband signal processing apparatus 8 is provided for simplicity. Furthermore, means for receiving and demodulating a signal transmitted from the mobile station apparatus is omitted.
Operation of base station apparatus 1 in the above configuration when communicating with the mobile station apparatus will be explained below.
First, baseband signal generator 9 generates two baseband signals made up of an in-phase component (hereinafter referred to as “Ich”) and a quadrature component (hereinafter referred to as “Qch”) and outputs these components to radio apparatuses 4 and 5 via phase/amplitude correction sections 10 and 11. Baseband signal generator 9 also outputs a gain control signal to transmit power amplifiers 15 and 16 via phase/amplitude correction sections 10 and 11.
Here, the two baseband signals output to two radio apparatuses 4 and 5 are generated in baseband signal generator 9 by multiplying a same baseband signal by individual complex coefficients.
The baseband signals input to radio apparatuses 4 and 5 are subjected to quadrature modulation by quadrature modulation sections 13 and 14, up-converted to a radio frequency band, amplified by transmit power amplifiers 15 and 16 with an amplification gain controlled according to a gain control signal and become transmission signals.
These transmission signals are emitted from antennas 2 and 3 via switches 17 and 18 set so as to connect antenna 2 and transmit power amplifier 15 and connect antenna 3 and transmit power amplifier 16.
Here, by adjusting complex coefficients multiplied in baseband signal generator 9, it is possible to increase radiation field intensity only in a desired direction. This is called “providing transmission directivity”. Providing transmission directivity makes it possible to keep reception (SIR Signal to Interference Ratio) of other communication instruments high.
However, the characteristics of transmit power amplifiers 15 and 16 vary individually depending on variations of component analog elements. Since different unknown amplitude variations and phase rotations are added to the transmission signals of antennas 2 and 3, the transmission directivity formed in this way is different from the transmission directivity that is expected to be obtained by being multiplied by complex coefficients in baseband signal generator 9.
To prevent such a phenomenon, it is necessary to adjust transmit power amplifiers 15 and 16 so as to have identical characteristics. However, it is extremely difficult to accurately and time-invariably adjust the characteristics of analog elements such as amplifiers 15 and 16.
Thus, instead of adjusting the characteristics of transmit power amplifiers 15 and 16, a method of measuring and storing the characteristics of transmit power amplifiers 15 and 16 beforehand and correcting baseband signals during a communication anticipating that the amplitude and phase of transmission signals will change by the errors in the characteristics.
The characteristics of transmit power amplifiers 15 and 16 are measured beforehand before starting a communication. In this case, switch 17 is set so that transmit power amplifier 15 is connected with switch 6 and switch 18 is set so that transmit power amplifier 16 is connected with switch 6 first. Furthermore, switch 6 is set so that either switch 17 or 18 is connected with measuring apparatus 7. Here, switch 6 is set so that switch 17 is connected with measuring apparatus 7 first.
Then, in order to measure the characteristics of transmit power amplifiers 15 and 16, baseband signals with a known information symbol (in this case, especially referred to as “calibration signals”) are generated from baseband signal generator 9 and output to radio apparatuses 4 and 5.
The baseband signals input to radio apparatuses 4 and 5 are amplified by transmit power amplifiers 15 and 16 via quadrature modulation sections 13 and 14, and then output to measuring apparatus 7 via switches 17 and 6.
Then, measuring apparatus 7 measures the amplitude and phase of the input signal, calculates errors between these measured values and predetermined expected values of the amplitude and phase and these errors are stored in error storage section 12.
Hereinafter, switch 6 is switched so that switch 17 is connected with measuring apparatus 7 and the same processing above is carried out.
After this processing is completed, switches 17 and 18 are switched to the antennas 2 and 3 sides to start a communication. During this communication, phase/amplitude correction sections 10 and 11 correct baseband signals and gain control signals input from baseband signal generator 9 according to the errors stored in error storage section 12.
This correction is carried out by multiplying the baseband signals and gain control signals by complex coefficients that cancel out the characteristic errors of transmit power amplifiers 15 and 16. At this time, the complex coefficients multiplied on the baseband signals correct the phase of the transmission signals output from transmit power amplifiers 15 and 16 and the complex coefficients multiplied on the gain control signals correct the amplitude of the transmission signals.
Then, a configuration of the receiving side of the base station apparatus equipped with the conventional array antenna will be explained using the block diagram in FIG. 2.
Base station apparatus 51 shown in FIG. 2 is equipped with an array antenna made up of two antennas 52 and 53 and has a configuration equipped with radio apparatuses 54 and 55 with antennas 52 and 53 connected thereto, calibration signal generator 56 and baseband signal processing apparatus 57.
Radio apparatuses 54 and 55 have a configuration equipped with switches 58 and 59, AGC (Automatic Gain Control) amplifiers 60 and 61 and quadrature demodulation sections 62 and 63, respectively. Baseband signal processing apparatus 57 has a configuration equipped with phase/amplitude correction sections 64 and 65, baseband signal processing section 66 and error detection/storage section 67.
The following is an explanation of operation of base station apparatus 51 when receiving a signal from a mobile station apparatus.
However, when a signal sent from the mobile station apparatus is received, switch 58 is set so as to connect antenna 52 and AGC amplifier 60 and switch 59 is set so as to connect antenna 53 and AGC amplifier 61.
First, a signal received from antenna 52 is output to AGC amplifier 60 via switch 58 and AGC amplifier 60 performs auto-gain control so that the amplitude becomes constant.
In this case, an AGC signal indicating the auto-gain control result is output to baseband signal processing section 66 via phase/amplitude correction section 64 and the signal subjected to auto-gain control output from AGC amplifier 60 is demodulated by quadrature demodulation section 62 and becomes a baseband signal made up of an Ich and Qch. After the amplitude and phase of this baseband signal are corrected by phase/amplitude correction section 64, the baseband signal is output to baseband signal processing section 66. Furthermore, phase/amplitude correction section 64 also corrects the amplitude and phase of the AGC signal.
Baseband signal processing section 66 performs processing on the baseband signal and AGC signal such as conversion to a predetermined frequency. The same reception processing above is also performed on the system on the reception apparatus 55 side.
When such reception processing is carried out, as explained on the transmitting side above, radio apparatuses 54 and 55 have characteristic variations because AGC amplifiers 60 and 61 that make up radio apparatuses 54 and 55 are analog elements, and it is extremely difficult to adjust these variations accurately and time-invariably.
Thus, a method is adopted by which the characteristics of radio apparatuses 54 and 55 are measured and stored beforehand before starting reception processing and the baseband signal is corrected during reception anticipating that the baseband signal will change by the errors between these measured values and predetermined expected values of the amplitude and phase.
The method of measuring characteristics of radio apparatuses 54 and 55 will be described below.
Before carrying out this measurement, switch 58 is set so as to connect calibration signal generator 56 and AGC amplifier 60 and switch 59 is set so as to connect calibration signal generator 56 and AGC amplifier 61.
Then, in order to measure characteristics of radio apparatuses 54 and 55, a calibration signal with a known information symbol is generated from calibration signal generator 56 and this signal is output to baseband signal processing section 66 via radio apparatuses 54 and 55 and phase/amplitude correction sections 64 and 65 and further output to error detection/storage section 67.
Error detection/storage section 67 detects the amplitude and phase of the base band signal and AGC signal based on the calibration signal, finds errors between these detected values and the predetermined expected values of the amplitude and phase and stores these errors.
Then, switches 58 and 59 are switched to the antenna 52 and 53 sides to start reception. During reception, phase/amplitude correction sections 64 and 65 correct the baseband signals and AGC signals of their respective systems according to the errors stored in error detection/storage section 67.
This correction is performed by multiplying the baseband signals and AGC signals by complex coefficients according to the aforementioned errors that cancel out the characteristic errors of radio apparatuses 54 and 55.
However, the conventional apparatus has a problem that it is unable, during transmission, to carry out a measurement to find characteristic errors of transmit power amplifiers 15 and 16 necessary to correct amplitude and phase shifts of the transmission signal during a communication with the mobile station apparatus and the communication must be interrupted to carry out the measurement.
Likewise, the conventional apparatus has another problem that it is unable, during reception, to carry out a measurement to find characteristic errors of radio apparatuses 54 and 55 necessary to correct amplitude and phase shifts of the baseband signals and AGC signals during a communication with the mobile station apparatus and the communication must be interrupted to carry out the measurement.
Moreover, carrying out the measurement above requires provision of an oscillation circuit to generate a calibration signal with a known information symbol on the transmitting side, which involves a problem of increasing the size and cost of the apparatus accordingly. Likewise, it also requires provision of an oscillation circuit to generate a calibration signal on the receiving side, which involves a problem of increasing the size and cost of the apparatus accordingly.